The long-term objectives of this research is to improve the present understanding of the hormonal control of male reproductive functions by elucidating the effects of prolactin (PRL) on the testes and the hypothalamic-pituitary system. Benefits from this project will include obtaining new information on the control of endocrine and reproductive functions in health and disease. One of the specific aims for the next five years will be to define hormonal control of testicular PRL receptors before and after sexual maturation and to relate the PRL-induced changes in the levels of LH and PRL receptors to the ability of the testis to respond to LH and PRL stimulation. Other aims of this project will be to elucidate the mechanisms responsible for the effects of PRL on the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. The proposed studies will involve measurements of binding of radioactively labelled hormones to testicular membrane preparations, measurements of blood levels of pituitary and testicular hormones, assessment of noradrenergic, dopaminergic and serotonergic transmission in the hypothalamus by measurement of levels, turnover rates and metabolites of neurotransmitters in different brain regions, determinations of luteinizing hormone releasing hormone (LHR) stores in the hypothalamus and studying the control of its release in vivo and in vitro, as well as measurements of pituitary LHRH receptors and pituitary responsiveness to LHRH stimulation. Most of the proposed studies will be conducted in golden hamsters because in this species, PRL has already been shown to exert major and physiologically important effects on male reproductive functions, and PRL release can be readily manipulated by environmental lighting conditions without pharmacologic or surgical intervention. However, in order to more readily identify cause-effect relationships between the various effects of PRL and to determine whether our findings may apply to other mammalian species, we will conduct parallel comparative studies in the rat, the mouse and the Djungarian hamster.